Detection of contaminants such as pollutants, toxins, poisons, and biological agents is critically important in many industrial, public, and private environments. Accordingly, a variety of environmental sensors have been developed. These environmental sensors are generally large enough to be handheld or mounted in the areas being monitored. Unfortunately, the size and need for separate mechanical and electrical components make these environmental sensors expensive when compared to the costs of integrated circuits. Sensors that could be manufactured using nanotechnology could potentially reduce sensing costs and permit new sensing capabilities, for example, for environments that are difficult to access or that have insufficient space to accommodate conventional sensors.
One group of sensors known in biomedical applications is the fiber-optic evanescent fluorescence sensors. These sensors generally sense or measure the concentrations of target molecules that are known to absorb light having a first wavelength λ and to subsequently fluoresce by emitting light having a second wavelength λ′. Such sensors typical include an optical fiber that is inserted into a liquid containing the target molecules, while light having wavelength λ is directed through the optical fiber. The target molecules that are within the evanescent field surrounding the optical fiber can then absorb light of wavelength λ from the optical fiber and subsequently fluoresce to emit back into the optical fiber light having wavelength λ′. A detector coupled to the optical fiber measures the intensity of the light having frequency λ′, and that measurement indicates the presence or number of target molecules within the evanescent field of the optical fiber.
Current evanescent fluorescence sensors have a number of drawbacks. In particular, such sensors are relatively large and limited to sensing target molecules that have suitable fluorescent properties. Further, evanescent fluorescence sensors are typically limited to sensing target molecules in a liquid because molecules in a gas at room temperature spend only a short time within the evanescent field, i.e., within a distance of about λ/4 of the optical fiber, and therefore generally move away from the optical fiber before fluorescing.
In view of the limitations of current environmental sensors, inexpensive sensors and sensing methods for detecting a variety of contaminant species in a gas or a liquid are needed.